openstack/swift
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Erasure Code Documentation

Browse Source 
This patch adds all the relevant EC documentation to
the source tree. Notable additions are:
  - Updated SAIO documentation
  - Updates to existing swift documentation; and
  - Erasure Coding overview

Co-Authored-By: Alistair Coles <alistair.coles@hp.com>
Co-Authored-By: Thiago da Silva <thiago@redhat.com>
Co-Authored-By: John Dickinson <me@not.mn>
Co-Authored-By: Clay Gerrard <clay.gerrard@gmail.com>
Co-Authored-By: Tushar Gohad <tushar.gohad@intel.com>
Co-Authored-By: Samuel Merritt <sam@swiftstack.com>
Co-Authored-By: Christian Schwede <christian.schwede@enovance.com>
Co-Authored-By: Yuan Zhou <yuan.zhou@intel.com>
Change-Id: I0403016a4bb7dad9535891632753b0e5e9d402eb
Implements: blueprint swift-ec
Signed-off-by: Thiago da Silva <thiago@redhat.com>
This commit is contained in:
Paul Luse 2014-07-03 15:59:33 -07:00 committed by Clay Gerrard
parent 647b66a2ce
commit 8f5d4d2455
15 changed files with 943 additions and 147 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
doc/saio/bin/remakerings
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@ -16,6 +16,16 @@ swift-ring-builder object-1.builder add r1z2-127.0.0.1:6020/sdb2 1
swift-ring-builder object-1.builder add r1z3-127.0.0.1:6030/sdb3 1
swift-ring-builder object-1.builder add r1z4-127.0.0.1:6040/sdb4 1
swift-ring-builder object-1.builder rebalance
swift-ring-builder object-2.builder create 10 6 1
swift-ring-builder object-2.builder add r1z1-127.0.0.1:6010/sdb1 1
swift-ring-builder object-2.builder add r1z1-127.0.0.1:6010/sdb5 1
swift-ring-builder object-2.builder add r1z2-127.0.0.1:6020/sdb2 1
swift-ring-builder object-2.builder add r1z2-127.0.0.1:6020/sdb6 1
swift-ring-builder object-2.builder add r1z3-127.0.0.1:6030/sdb3 1
swift-ring-builder object-2.builder add r1z3-127.0.0.1:6030/sdb7 1
swift-ring-builder object-2.builder add r1z4-127.0.0.1:6040/sdb4 1
swift-ring-builder object-2.builder add r1z4-127.0.0.1:6040/sdb8 1
swift-ring-builder object-2.builder rebalance
swift-ring-builder container.builder create 10 3 1
swift-ring-builder container.builder add r1z1-127.0.0.1:6011/sdb1 1
swift-ring-builder container.builder add r1z2-127.0.0.1:6021/sdb2 1

5
doc/saio/bin/resetswift
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@ -9,7 +9,10 @@ sudo mkfs.xfs -f ${SAIO_BLOCK_DEVICE:-/dev/sdb1}
sudo mount /mnt/sdb1
sudo mkdir /mnt/sdb1/1 /mnt/sdb1/2 /mnt/sdb1/3 /mnt/sdb1/4
sudo chown ${USER}:${USER} /mnt/sdb1/*
mkdir -p /srv/1/node/sdb1 /srv/2/node/sdb2 /srv/3/node/sdb3 /srv/4/node/sdb4
mkdir -p /srv/1/node/sdb1 /srv/1/node/sdb5 \
/srv/2/node/sdb2 /srv/2/node/sdb6 \
/srv/3/node/sdb3 /srv/3/node/sdb7 \
/srv/4/node/sdb4 /srv/4/node/sdb8
sudo rm -f /var/log/debug /var/log/messages /var/log/rsyncd.log /var/log/syslog
find /var/cache/swift* -type f -name *.recon -exec rm -f {} \;
# On Fedora use "systemctl restart <service>"

2
doc/saio/swift/object-server/1.conf
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@ -22,6 +22,8 @@ use = egg:swift#recon
[object-replicator]
vm_test_mode = yes
[object-reconstructor]
[object-updater]
[object-auditor]

2
doc/saio/swift/object-server/2.conf
View File

@ -22,6 +22,8 @@ use = egg:swift#recon
[object-replicator]
vm_test_mode = yes
[object-reconstructor]
[object-updater]
[object-auditor]

2
doc/saio/swift/object-server/3.conf
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@ -22,6 +22,8 @@ use = egg:swift#recon
[object-replicator]
vm_test_mode = yes
[object-reconstructor]
[object-updater]
[object-auditor]

2
doc/saio/swift/object-server/4.conf
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@ -22,6 +22,8 @@ use = egg:swift#recon
[object-replicator]
vm_test_mode = yes
[object-reconstructor]
[object-updater]
[object-auditor]

9
doc/saio/swift/swift.conf
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@ -5,7 +5,16 @@ swift_hash_path_suffix = changeme
[storage-policy:0]
name = gold
policy_type = replication
default = yes
[storage-policy:1]
name = silver
policy_type = replication
[storage-policy:2]
name = ec42
policy_type = erasure_coding
ec_type = jerasure_rs_vand
ec_num_data_fragments = 4
ec_num_parity_fragments = 2

2
doc/source/associated_projects.rst
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@ -104,5 +104,7 @@ Other
* `Swiftsync <https://github.com/stackforge/swiftsync>`_ - A massive syncer between two swift clusters.
* `Django Swiftbrowser <https://github.com/cschwede/django-swiftbrowser>`_ - Simple Django web app to access Openstack Swift.
* `Swift-account-stats <https://github.com/enovance/swift-account-stats>`_ - Swift-account-stats is a tool to report statistics on Swift usage at tenant and global levels.
* `PyECLib <https://bitbucket.org/kmgreen2/pyeclib>`_ - High Level Erasure Code library used by Swift
* `liberasurecode <http://www.bytebucket.org/tsg-/liberasurecode>`_ - Low Level Erasure Code library used by PyECLib
* `Swift Browser <https://github.com/zerovm/swift-browser>`_ - JavaScript interface for Swift
* `swift-ui <https://github.com/fanatic/swift-ui>`_ - OpenStack Swift web browser

40
doc/source/development_saio.rst
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@ -87,8 +87,11 @@ another device when creating the VM, and follow these instructions:
sudo chown ${USER}:${USER} /mnt/sdb1/*
sudo mkdir /srv
for x in {1..4}; do sudo ln -s /mnt/sdb1/$x /srv/$x; done
sudo mkdir -p /srv/1/node/sdb1 /srv/2/node/sdb2 /srv/3/node/sdb3 \
/srv/4/node/sdb4 /var/run/swift
sudo mkdir -p /srv/1/node/sdb1 /srv/1/node/sdb5 \
/srv/2/node/sdb2 /srv/2/node/sdb6 \
/srv/3/node/sdb3 /srv/3/node/sdb7 \
/srv/4/node/sdb4 /srv/4/node/sdb8 \
/var/run/swift
sudo chown -R ${USER}:${USER} /var/run/swift
# **Make sure to include the trailing slash after /srv/$x/**
for x in {1..4}; do sudo chown -R ${USER}:${USER} /srv/$x/; done
@ -124,7 +127,11 @@ these instructions:
sudo mkdir /mnt/sdb1/1 /mnt/sdb1/2 /mnt/sdb1/3 /mnt/sdb1/4
sudo chown ${USER}:${USER} /mnt/sdb1/*
for x in {1..4}; do sudo ln -s /mnt/sdb1/$x /srv/$x; done
sudo mkdir -p /srv/1/node/sdb1 /srv/2/node/sdb2 /srv/3/node/sdb3 /srv/4/node/sdb4 /var/run/swift
sudo mkdir -p /srv/1/node/sdb1 /srv/1/node/sdb5 \
/srv/2/node/sdb2 /srv/2/node/sdb6 \
/srv/3/node/sdb3 /srv/3/node/sdb7 \
/srv/4/node/sdb4 /srv/4/node/sdb8 \
/var/run/swift
sudo chown -R ${USER}:${USER} /var/run/swift
# **Make sure to include the trailing slash after /srv/$x/**
for x in {1..4}; do sudo chown -R ${USER}:${USER} /srv/$x/; done
@ -402,7 +409,7 @@ Setting up scripts for running Swift
#. Copy the SAIO scripts for resetting the environment::
cd $HOME/swift/doc; cp -r saio/bin $HOME/bin; cd -
cd $HOME/swift/doc; cp saio/bin/* $HOME/bin; cd -
chmod +x $HOME/bin/*
#. Edit the ``$HOME/bin/resetswift`` script
@ -455,30 +462,41 @@ Setting up scripts for running Swift
.. literalinclude:: /../saio/bin/remakerings
You can expect the output from this command to produce the following (note
that 2 object rings are created in order to test storage policies in the
SAIO environment however they map to the same nodes)::
You can expect the output from this command to produce the following. Note
that 3 object rings are created in order to test storage policies and EC in
the SAIO environment. The EC ring is the only one with all 8 devices.
There are also two replication rings, one for 3x replication and another
for 2x replication, but those rings only use 4 devices::
Device d0r1z1-127.0.0.1:6010R127.0.0.1:6010/sdb1_"" with 1.0 weight got id 0
Device d1r1z2-127.0.0.1:6020R127.0.0.1:6020/sdb2_"" with 1.0 weight got id 1
Device d2r1z3-127.0.0.1:6030R127.0.0.1:6030/sdb3_"" with 1.0 weight got id 2
Device d3r1z4-127.0.0.1:6040R127.0.0.1:6040/sdb4_"" with 1.0 weight got id 3
Reassigned 1024 (100.00%) partitions. Balance is now 0.00.
Reassigned 1024 (100.00%) partitions. Balance is now 0.00. Dispersion is now 0.00
Device d0r1z1-127.0.0.1:6010R127.0.0.1:6010/sdb1_"" with 1.0 weight got id 0
Device d1r1z2-127.0.0.1:6020R127.0.0.1:6020/sdb2_"" with 1.0 weight got id 1
Device d2r1z3-127.0.0.1:6030R127.0.0.1:6030/sdb3_"" with 1.0 weight got id 2
Device d3r1z4-127.0.0.1:6040R127.0.0.1:6040/sdb4_"" with 1.0 weight got id 3
Reassigned 1024 (100.00%) partitions. Balance is now 0.00.
Reassigned 1024 (100.00%) partitions. Balance is now 0.00. Dispersion is now 0.00
Device d0r1z1-127.0.0.1:6010R127.0.0.1:6010/sdb1_"" with 1.0 weight got id 0
Device d1r1z1-127.0.0.1:6010R127.0.0.1:6010/sdb5_"" with 1.0 weight got id 1
Device d2r1z2-127.0.0.1:6020R127.0.0.1:6020/sdb2_"" with 1.0 weight got id 2
Device d3r1z2-127.0.0.1:6020R127.0.0.1:6020/sdb6_"" with 1.0 weight got id 3
Device d4r1z3-127.0.0.1:6030R127.0.0.1:6030/sdb3_"" with 1.0 weight got id 4
Device d5r1z3-127.0.0.1:6030R127.0.0.1:6030/sdb7_"" with 1.0 weight got id 5
Device d6r1z4-127.0.0.1:6040R127.0.0.1:6040/sdb4_"" with 1.0 weight got id 6
Device d7r1z4-127.0.0.1:6040R127.0.0.1:6040/sdb8_"" with 1.0 weight got id 7
Reassigned 1024 (100.00%) partitions. Balance is now 0.00. Dispersion is now 0.00
Device d0r1z1-127.0.0.1:6011R127.0.0.1:6011/sdb1_"" with 1.0 weight got id 0
Device d1r1z2-127.0.0.1:6021R127.0.0.1:6021/sdb2_"" with 1.0 weight got id 1
Device d2r1z3-127.0.0.1:6031R127.0.0.1:6031/sdb3_"" with 1.0 weight got id 2
Device d3r1z4-127.0.0.1:6041R127.0.0.1:6041/sdb4_"" with 1.0 weight got id 3
Reassigned 1024 (100.00%) partitions. Balance is now 0.00.
Reassigned 1024 (100.00%) partitions. Balance is now 0.00. Dispersion is now 0.00
Device d0r1z1-127.0.0.1:6012R127.0.0.1:6012/sdb1_"" with 1.0 weight got id 0
Device d1r1z2-127.0.0.1:6022R127.0.0.1:6022/sdb2_"" with 1.0 weight got id 1
Device d2r1z3-127.0.0.1:6032R127.0.0.1:6032/sdb3_"" with 1.0 weight got id 2
Device d3r1z4-127.0.0.1:6042R127.0.0.1:6042/sdb4_"" with 1.0 weight got id 3
Reassigned 1024 (100.00%) partitions. Balance is now 0.00.
Reassigned 1024 (100.00%) partitions. Balance is now 0.00. Dispersion is now 0.00
#. Read more about Storage Policies and your SAIO :doc:`policies_saio`

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1
doc/source/index.rst
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@ -56,6 +56,7 @@ Overview and Concepts
overview_expiring_objects
cors
crossdomain
overview_erasure_code
overview_backing_store
associated_projects

17
doc/source/overview_architecture.rst
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@ -11,7 +11,10 @@ Proxy Server
The Proxy Server is responsible for tying together the rest of the Swift
architecture. For each request, it will look up the location of the account,
container, or object in the ring (see below) and route the request accordingly.
The public API is also exposed through the Proxy Server.
For Erasure Code type policies, the Proxy Server is also responsible for
encoding and decoding object data. See :doc:`overview_erasure_code` for
complete information on Erasure Code suport. The public API is also exposed
through the Proxy Server.
A large number of failures are also handled in the Proxy Server. For
example, if a server is unavailable for an object PUT, it will ask the
@ -87,7 +90,8 @@ implementing a particular differentiation.
For example, one might have the default policy with 3x replication, and create
a second policy which, when applied to new containers only uses 2x replication.
Another might add SSDs to a set of storage nodes and create a performance tier
storage policy for certain containers to have their objects stored there.
storage policy for certain containers to have their objects stored there. Yet
another might be the use of Erasure Coding to define a cold-storage tier.
This mapping is then exposed on a per-container basis, where each container
can be assigned a specific storage policy when it is created, which remains in
@ -156,6 +160,15 @@ item (object, container, or account) is deleted, a tombstone is set as the
latest version of the item. The replicator will see the tombstone and ensure
that the item is removed from the entire system.
--------------
Reconstruction
--------------
The reconstructor is used by Erasure Code policies and is analogous to the
replicator for Replication type policies. See :doc:`overview_erasure_code`
for complete information on both Erasure Code support as well as the
reconstructor.
--------
Updaters
--------

672
doc/source/overview_erasure_code.rst Executable file
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@ -0,0 +1,672 @@
====================
Erasure Code Support
====================
--------------------------
Beta: Not production ready
--------------------------
The erasure code support in Swift is considered "beta" at this point.
Most major functionality is included, but it has not been tested or validated
at large scale. This feature relies on ssync for durability. Deployers are
urged to do extensive testing and not deploy production data using an
erasure code storage policy.
If any bugs are found during testing, please report them to
https://bugs.launchpad.net/swift
-------------------------------
History and Theory of Operation
-------------------------------
There's a lot of good material out there on Erasure Code (EC) theory, this short
introduction is just meant to provide some basic context to help the reader
better understand the implementation in Swift.
Erasure Coding for storage applications grew out of Coding Theory as far back as
the 1960s with the Reed-Solomon codes. These codes have been used for years in
applications ranging from CDs to DVDs to general communications and, yes, even
in the space program starting with Voyager! The basic idea is that some amount
of data is broken up into smaller pieces called fragments and coded in such a
way that it can be transmitted with the ability to tolerate the loss of some
number of the coded fragments. That's where the word "erasure" comes in, if you
transmit 14 fragments and only 13 are received then one of them is said to be
"erased". The word "erasure" provides an important distinction with EC; it
isn't about detecting errors, it's about dealing with failures. Another
important element of EC is that the number of erasures that can be tolerated can
be adjusted to meet the needs of the application.
At a high level EC works by using a specific scheme to break up a single data
buffer into several smaller data buffers then, depending on the scheme,
performing some encoding operation on that data in order to generate additional
information. So you end up with more data than you started with and that extra
data is often called "parity". Note that there are many, many different
encoding techniques that vary both in how they organize and manipulate the data
as well by what means they use to calculate parity. For example, one scheme
might rely on `Galois Field Arithmetic <http://www.ssrc.ucsc.edu/Papers/plank-
fast13.pdf>`_ while others may work with only XOR. The number of variations and
details about their differences are well beyond the scope of this introduction,
but we will talk more about a few of them when we get into the implementation of
EC in Swift.
--------------------------------
Overview of EC Support in Swift
--------------------------------
First and foremost, from an application perspective EC support is totally
transparent. There are no EC related external API; a container is simply created
using a Storage Policy defined to use EC and then interaction with the cluster
is the same as any other durability policy.
EC is implemented in Swift as a Storage Policy, see :doc:`overview_policies` for
complete details on Storage Policies. Because support is implemented as a
Storage Policy, all of the storage devices associated with your cluster's EC
capability can be isolated. It is entirely possible to share devices between
storage policies, but for EC it may make more sense to not only use separate
devices but possibly even entire nodes dedicated for EC.
Which direction one chooses depends on why the EC policy is being deployed. If,
for example, there is a production replication policy in place already and the
goal is to add a cold storage tier such that the existing nodes performing
replication are impacted as little as possible, adding a new set of nodes
dedicated to EC might make the most sense but also incurs the most cost. On the
other hand, if EC is being added as a capability to provide additional
durability for a specific set of applications and the existing infrastructure is
well suited for EC (sufficient number of nodes, zones for the EC scheme that is
chosen) then leveraging the existing infrastructure such that the EC ring shares
nodes with the replication ring makes the most sense. These are some of the
main considerations:
* Layout of existing infrastructure.
* Cost of adding dedicated EC nodes (or just dedicated EC devices).
* Intended usage model(s).
The Swift code base does not include any of the algorithms necessary to perform
the actual encoding and decoding of data; that is left to external libraries.
The Storage Policies architecture is leveraged to enable EC on a per container
basis -- the object rings are still used to determine the placement of EC data
fragments. Although there are several code paths that are unique to an operation
associated with an EC policy, an external dependency to an Erasure Code library
is what Swift counts on to perform the low level EC functions. The use of an
external library allows for maximum flexibility as there are a significant
number of options out there, each with its owns pros and cons that can vary
greatly from one use case to another.
---------------------------------------
PyECLib: External Erasure Code Library
---------------------------------------
PyECLib is a Python Erasure Coding Library originally designed and written as
part of the effort to add EC support to the Swift project, however it is an
independent project. The library provides a well-defined and simple Python
interface and internally implements a plug-in architecture allowing it to take
advantage of many well-known C libraries such as:
* Jerasure and GFComplete at http://jerasure.org.
* Intel(R) ISA-L at http://01.org/intel%C2%AE-storage-acceleration-library-open-source-version.
* Or write your own!
PyECLib uses a C based library called liberasurecode to implement the plug in
infrastructure; liberasure code is available at:
* liberasurecode: https://bitbucket.org/tsg-/liberasurecode
PyECLib itself therefore allows for not only choice but further extensibility as
well. PyECLib also comes with a handy utility to help determine the best
algorithm to use based on the equipment that will be used (processors and server
configurations may vary in performance per algorithm). More on this will be
covered in the configuration section. PyECLib is included as a Swift
requirement.
For complete details see `PyECLib <https://bitbucket.org/kmgreen2/pyeclib>`_
------------------------------
Storing and Retrieving Objects
------------------------------
We will discuss the details of how PUT and GET work in the "Under the Hood"
section later on. The key point here is that all of the erasure code work goes
on behind the scenes; this summary is a high level information overview only.
The PUT flow looks like this:
#. The proxy server streams in an object and buffers up "a segment" of data
(size is configurable).
#. The proxy server calls on PyECLib to encode the data into smaller fragments.
#. The proxy streams the encoded fragments out to the storage nodes based on
ring locations.
#. Repeat until the client is done sending data.
#. The client is notified of completion when a quorum is met.
The GET flow looks like this:
#. The proxy server makes simultaneous requests to participating nodes.
#. As soon as the proxy has the fragments it needs, it calls on PyECLib to
decode the data.
#. The proxy streams the decoded data it has back to the client.
#. Repeat until the proxy is done sending data back to the client.
It may sound like, from this high level overview, that using EC is going to
cause an explosion in the number of actual files stored in each node's local
file system. Although it is true that more files will be stored (because an
object is broken into pieces), the implementation works to minimize this where
possible, more details are available in the Under the Hood section.
-------------
Handoff Nodes
-------------
In EC policies, similarly to replication, handoff nodes are a set of storage
nodes used to augment the list of primary nodes responsible for storing an
erasure coded object. These handoff nodes are used in the event that one or more
of the primaries are unavailable. Handoff nodes are still selected with an
attempt to achieve maximum separation of the data being placed.
--------------
Reconstruction
--------------
For an EC policy, reconstruction is analogous to the process of replication for
a replication type policy -- essentially "the reconstructor" replaces "the
replicator" for EC policy types. The basic framework of reconstruction is very
similar to that of replication with a few notable exceptions:
* Because EC does not actually replicate partitions, it needs to operate at a
finer granularity than what is provided with rsync, therefore EC leverages
much of ssync behind the scenes (you do not need to manually configure ssync).
* Once a pair of nodes has determined the need to replace a missing object
fragment, instead of pushing over a copy like replication would do, the
reconstructor has to read in enough surviving fragments from other nodes and
perform a local reconstruction before it has the correct data to push to the
other node.
* A reconstructor does not talk to all other reconstructors in the set of nodes
responsible for an EC partition, this would be far too chatty, instead each
reconstructor is responsible for sync'ing with the partition's closest two
neighbors (closest meaning left and right on the ring).
.. note::
EC work (encode and decode) takes place both on the proxy nodes, for PUT/GET
operations, as well as on the storage nodes for reconstruction. As with
replication, reconstruction can be the result of rebalancing, bit-rot, drive
failure or reverting data from a hand-off node back to its primary.
--------------------------
Performance Considerations
--------------------------
Efforts are underway to characterize performance of various Erasure Code
schemes. One of the main goals of the beta release is to perform this
characterization and encourage others to do so and provide meaningful feedback
to the development community. There are many factors that will affect
performance of EC so it is vital that we have multiple characterization
activities happening.
In general, EC has different performance characteristics than replicated data.
EC requires substantially more CPU to read and write data, and is more suited
for larger objects that are not frequently accessed (eg backups).
----------------------------
Using an Erasure Code Policy
----------------------------
To use an EC policy, the administrator simply needs to define an EC policy in
`swift.conf` and create/configure the associated object ring. An example of how
an EC policy can be setup is shown below::
[storage-policy:2]
name = ec104
policy_type = erasure_coding
ec_type = jerasure_rs_vand
ec_num_data_fragments = 10
ec_num_parity_fragments = 4
ec_object_segment_size = 1048576
Let's take a closer look at each configuration parameter:
* ``name``: This is a standard storage policy parameter.
See :doc:`overview_policies` for details.
* ``policy_type``: Set this to ``erasure_coding`` to indicate that this is an EC
policy.
* ``ec_type``: Set this value according to the available options in the selected
PyECLib back-end. This specifies the EC scheme that is to be used. For
example the option shown here selects Vandermonde Reed-Solomon encoding while
an option of ``flat_xor_hd_3`` would select Flat-XOR based HD combination
codes. See the `PyECLib <https://bitbucket.org/kmgreen2/pyeclib>`_ page for
full details.
* ``ec_num_data_fragments``: The total number of fragments that will be
comprised of data.
* ``ec_num_parity_fragments``: The total number of fragments that will be
comprised of parity.
* ``ec_object_segment_size``: The amount of data that will be buffered up before
feeding a segment into the encoder/decoder. The default value is 1048576.
When PyECLib encodes an object, it will break it into N fragments. However, what
is important during configuration, is how many of those are data and how many
are parity. So in the example above, PyECLib will actually break an object in
14 different fragments, 10 of them will be made up of actual object data and 4
of them will be made of parity data (calculations depending on ec_type).
When deciding which devices to use in the EC policy's object ring, be sure to
carefully consider the performance impacts. Running some performance
benchmarking in a test environment for your configuration is highly recommended
before deployment. Once you have configured your EC policy in `swift.conf` and
created your object ring, your application is ready to start using EC simply by
creating a container with the specified policy name and interacting as usual.
.. note::
It's important to note that once you have deployed a policy and have created
objects with that policy, these configurations options cannot be changed. In
case a change in the configuration is desired, you must create a new policy
and migrate the data to a new container.
Migrating Between Policies
--------------------------
A common usage of EC is to migrate less commonly accessed data from a more
expensive but lower latency policy such as replication. When an application
determines that it wants to move data from a replication policy to an EC policy,
it simply needs to move the data from the replicated container to an EC
container that was created with the target durability policy.
Region Support
--------------
For at least the initial version of EC, it is not recommended that an EC scheme
span beyond a single region, neither performance nor functional validation has
be been done in such a configuration.
--------------
Under the Hood
--------------
Now that we've explained a little about EC support in Swift and how to
configure/use it, let's explore how EC fits in at the nuts-n-bolts level.
Terminology
-----------
The term 'fragment' has been used already to describe the output of the EC
process (a series of fragments) however we need to define some other key terms
here before going any deeper. Without paying special attention to using the
correct terms consistently, it is very easy to get confused in a hurry!
* **chunk**: HTTP chunks received over wire (term not used to describe any EC
specific operation).
* **segment**: Not to be confused with SLO/DLO use of the word, in EC we call a
segment a series of consecutive HTTP chunks buffered up before performing an
EC operation.
* **fragment**: Data and parity 'fragments' are generated when erasure coding
transformation is applied to a segment.
* **EC archive**: A concatenation of EC fragments; to a storage node this looks
like an object.
* **ec_ndata**: Number of EC data fragments.
* **ec_nparity**: Number of EC parity fragments.
Middleware
----------
Middleware remains unchanged. For most middleware (e.g., SLO/DLO) the fact that
the proxy is fragmenting incoming objects is transparent. For list endpoints,
however, it is a bit different. A caller of list endpoints will get back the
locations of all of the fragments. The caller will be unable to re-assemble the
original object with this information, however the node locations may still
prove to be useful information for some applications.
On Disk Storage
---------------
EC archives are stored on disk in their respective objects-N directory based on
their policy index. See :doc:`overview_policies` for details on per policy
directory information.
The actual names on disk of EC archives also have one additional piece of data
encoded in the filename, the fragment archive index.
Each storage policy now must include a transformation function that diskfile
will use to build the filename to store on disk. The functions are implemented
in the diskfile module as policy specific sub classes ``DiskFileManager``.
This is required for a few reasons. For one, it allows us to store fragment
archives of different indexes on the same storage node which is not typical
however it is possible in many circumstances. Without unique filenames for the
different EC archive files in a set, we would be at risk of overwriting one
archive of index n with another of index m in some scenarios.
The transformation function for the replication policy is simply a NOP. For
reconstruction, the index is appended to the filename just before the .data
extension. An example filename for a fragment archive storing the 5th fragment
would like this this::
1418673556.92690#5.data
An additional file is also included for Erasure Code policies called the
``.durable`` file. Its meaning will be covered in detail later, however, its on-
disk format does not require the name transformation function that was just
covered. The .durable for the example above would simply look like this::
1418673556.92690.durable
And it would be found alongside every fragment specific .data file following a
100% successful PUT operation.
Proxy Server
------------
High Level
==========
The Proxy Server handles Erasure Coding in a different manner than replication,
therefore there are several code paths unique to EC policies either though sub
classing or simple conditionals. Taking a closer look at the PUT and the GET
paths will help make this clearer. But first, a high level overview of how an
object flows through the system:
.. image:: images/ec_overview.png
Note how:
* Incoming objects are buffered into segments at the proxy.
* Segments are erasure coded into fragments at the proxy.
* The proxy stripes fragments across participating nodes such that the on-disk
stored files that we call a fragment archive is appended with each new
fragment.
This scheme makes it possible to minimize the number of on-disk files given our
segmenting and fragmenting.
Multi_Phase Conversation
========================
Multi-part MIME document support is used to allow the proxy to engage in a
handshake conversation with the storage node for processing PUT requests. This
is required for a few different reasons.
#. From the perspective of the storage node, a fragment archive is really just
another object, we need a mechanism to send down the original object etag
after all fragment archives have landed.
#. Without introducing strong consistency semantics, the proxy needs a mechanism
to know when a quorum of fragment archives have actually made it to disk
before it can inform the client of a successful PUT.
MIME supports a conversation between the proxy and the storage nodes for every
PUT. This provides us with the ability to handle a PUT in one connection and
assure that we have the essence of a 2 phase commit, basically having the proxy
communicate back to the storage nodes once it has confirmation that all fragment
archives in the set have been committed. Note that we still require a quorum of
data elements of the conversation to complete before signaling status to the
client but we can relax that requirement for the commit phase such that only 2
confirmations to that phase of the conversation are required for success as the
reconstructor will assure propagation of markers that indicate data durability.
This provides the storage node with a cheap indicator of the last known durable
set of fragment archives for a given object on a successful durable PUT, this is
known as the ``.durable`` file. The presence of a ``.durable`` file means, to
the object server, `there is a set of ts.data files that are durable at
timestamp ts.` Note that the completion of the commit phase of the conversation
is also a signal for the object server to go ahead and immediately delete older
timestamp files for this object. This is critical as we do not want to delete
the older object until the storage node has confirmation from the proxy, via the
multi-phase conversation, that the other nodes have landed enough for a quorum.
The basic flow looks like this:
* The Proxy Server erasure codes and streams the object fragments
(ec_ndata + ec_nparity) to the storage nodes.
* The storage nodes store objects as EC archives and upon finishing object
data/metadata write, send a 1st-phase response to proxy.
* Upon quorum of storage nodes responses, the proxy initiates 2nd-phase by
sending commit confirmations to object servers.
* Upon receipt of commit message, object servers store a 0-byte data file as
`<timestamp>.durable` indicating successful PUT, and send a final response to
the proxy server.
* The proxy waits for a minimal number of two object servers to respond with a
success (2xx) status before responding to the client with a successful
status. In this particular case it was decided that two responses was
the mininum amount to know that the file would be propagated in case of
failure from other others and because a greater number would potentially
mean more latency, which should be avoided if possible.
Here is a high level example of what the conversation looks like::
proxy: PUT /p/a/c/o
Transfer-Encoding': 'chunked'
Expect': '100-continue'
X-Backend-Obj-Multiphase-Commit: yes
obj: 100 Continue
X-Obj-Multiphase-Commit: yes
proxy: --MIMEboundary
X-Document: object body
<obj_data>
--MIMEboundary
X-Document: object metadata
Content-MD5: <footer_meta_cksum>
<footer_meta>
--MIMEboundary
<object server writes data, metadata>
obj: 100 Continue
<quorum>
proxy: X-Document: put commit
commit_confirmation
--MIMEboundary--
<object server writes ts.durable state>
obj: 20x
<proxy waits to receive >=2 2xx responses>
proxy: 2xx -> client
A few key points on the .durable file:
* The .durable file means \"the matching .data file for this has sufficient
fragment archives somewhere, committed, to reconstruct the object\".
* The Proxy Server will never have knowledge, either on GET or HEAD, of the
existence of a .data file on an object server if it does not have a matching
.durable file.
* The object server will never return a .data that does not have a matching
.durable.
* When a proxy does a GET, it will only receive fragment archives that have
enough present somewhere to be reconstructed.
Partial PUT Failures
====================
A partial PUT failure has a few different modes. In one scenario the Proxy
Server is alive through the entire PUT conversation. This is a very
straightforward case. The client will receive a good response if and only if a
quorum of fragment archives were successfully landed on their storage nodes. In
this case the Reconstructor will discover the missing fragment archives, perform
a reconstruction and deliver fragment archives and their matching .durable files
to the nodes.
The more interesting case is what happens if the proxy dies in the middle of a
conversation. If it turns out that a quorum had been met and the commit phase
of the conversation finished, its as simple as the previous case in that the
reconstructor will repair things. However, if the commit didn't get a change to
happen then some number of the storage nodes have .data files on them (fragment
archives) but none of them knows whether there are enough elsewhere for the
entire object to be reconstructed. In this case the client will not have
received a 2xx response so there is no issue there, however, it is left to the
storage nodes to clean up the stale fragment archives. Work is ongoing in this
area to enable the proxy to play a role in reviving these fragment archives,
however, for the current release, a proxy failure after the start of a
conversation but before the commit message will simply result in a PUT failure.
GET
===
The GET for EC is different enough from replication that subclassing the
`BaseObjectController` to the `ECObjectController` enables an efficient way to
implement the high level steps described earlier:
#. The proxy server makes simultaneous requests to participating nodes.
#. As soon as the proxy has the fragments it needs, it calls on PyECLib to
decode the data.
#. The proxy streams the decoded data it has back to the client.
#. Repeat until the proxy is done sending data back to the client.
The GET path will attempt to contact all nodes participating in the EC scheme,
if not enough primaries respond then handoffs will be contacted just as with
replication. Etag and content length headers are updated for the client
response following reconstruction as the individual fragment archives metadata
is valid only for that fragment archive.
Object Server
-------------
The Object Server, like the Proxy Server, supports MIME conversations as
described in the proxy section earlier. This includes processing of the commit
message and decoding various sections of the MIME document to extract the footer
which includes things like the entire object etag.
DiskFile
========
Erasure code uses subclassed ``ECDiskFile``, ``ECDiskFileWriter`` and
``ECDiskFileManager`` to impement EC specific handling of on disk files. This
includes things like file name manipulation to include the fragment index in the
filename, determination of valid .data files based on .durable presence,
construction of EC specific hashes.pkl file to include fragment index
information, etc., etc.
Metadata
--------
There are few different categories of metadata that are associated with EC:
System Metadata: EC has a set of object level system metadata that it
attaches to each of the EC archives. The metadata is for internal use only:
* ``X-Object-Sysmeta-EC-Etag``: The Etag of the original object.
* ``X-Object-Sysmeta-EC-Content-Length``: The content length of the original
object.
* ``X-Object-Sysmeta-EC-Frag-Index``: The fragment index for the object.
* ``X-Object-Sysmeta-EC-Scheme``: Description of the EC policy used to encode
the object.
* ``X-Object-Sysmeta-EC-Segment-Size``: The segment size used for the object.
User Metadata: User metadata is unaffected by EC, however, a full copy of the
user metadata is stored with every EC archive. This is required as the
reconstructor needs this information and each reconstructor only communicates
with its closest neighbors on the ring.
PyECLib Metadata: PyECLib stores a small amount of metadata on a per fragment
basis. This metadata is not documented here as it is opaque to Swift.
Database Updates
----------------
As account and container rings are not associated with a Storage Policy, there
is no change to how these database updates occur when using an EC policy.
The Reconstructor
-----------------
The Reconstructor performs analogous functions to the replicator:
#. Recovery from disk drive failure.
#. Moving data around because of a rebalance.
#. Reverting data back to a primary from a handoff.
#. Recovering fragment archives from bit rot discovered by the auditor.
However, under the hood it operates quite differently. The following are some
of the key elements in understanding how the reconstructor operates.
Unlike the replicator, the work that the reconstructor does is not always as
easy to break down into the 2 basic tasks of synchronize or revert (move data
from handoff back to primary) because of the fact that one storage node can
house fragment archives of various indexes and each index really /"belongs/" to
a different node. So, whereas when the replicator is reverting data from a
handoff it has just one node to send its data to, the reconstructor can have
several. Additionally, its not always the case that the processing of a
particular suffix directory means one or the other for the entire directory (as
it does for replication). The scenarios that create these mixed situations can
be pretty complex so we will just focus on what the reconstructor does here and
not a detailed explanation of why.
Job Construction and Processing
===============================
Because of the nature of the work it has to do as described above, the
reconstructor builds jobs for a single job processor. The job itself contains
all of the information needed for the processor to execute the job which may be
a synchronization or a data reversion and there may be a mix of jobs that
perform both of these operations on the same suffix directory.
Jobs are constructed on a per partition basis and then per fragment index basis.
That is, there will be one job for every fragment index in a partition.
Performing this construction \"up front\" like this helps minimize the
interaction between nodes collecting hashes.pkl information.
Once a set of jobs for a partition has been constructed, those jobs are sent off
to threads for execution. The single job processor then performs the necessary
actions working closely with ssync to carry out its instructions. For data
reversion, the actual objects themselves are cleaned up via the ssync module and
once that partition's set of jobs is complete, the reconstructor will attempt to
remove the relevant directory structures.
The scenarios that job construction has to take into account include:
#. A partition directory with all fragment indexes matching the local node
index. This is the case where everything is where it belongs and we just
need to compare hashes and sync if needed, here we sync with our partners.
#. A partition directory with one local fragment index and mix of others. Here
we need to sync with our partners where fragment indexes matches the
local_id, all others are sync'd with their home nodes and then deleted.
#. A partition directory with no local fragment index and just one or more of
others. Here we sync with just the home nodes for the fragment indexes that
we have and then all the local archives are deleted. This is the basic
handoff reversion case.
.. note::
A \"home node\" is the node where the fragment index encoded in the
fragment archive's filename matches the node index of a node in the primary
partition list.
Node Communication
==================
The replicators talk to all nodes who have a copy of their object, typically
just 2 other nodes. For EC, having each reconstructor node talk to all nodes
would incur a large amount of overhead as there will typically be a much larger
number of nodes participating in the EC scheme. Therefore, the reconstructor is
built to talk to its adjacent nodes on the ring only. These nodes are typically
referred to as partners.
Reconstruction
==============
Reconstruction can be thought of sort of like replication but with an extra step
in the middle. The reconstructor is hard-wired to use ssync to determine what is
missing and desired by the other side. However, before an object is sent over
the wire it needs to be reconstructed from the remaining fragments as the local
fragment is just that - a different fragment index than what the other end is
asking for.
Thus, there are hooks in ssync for EC based policies. One case would be for
basic reconstruction which, at a high level, looks like this:
* Determine which nodes need to be contacted to collect other EC archives needed
to perform reconstruction.
* Update the etag and fragment index metadata elements of the newly constructed
fragment archive.
* Establish a connection to the target nodes and give ssync a DiskFileLike class
that it can stream data from.
The reader in this class gathers fragments from the nodes and uses PyECLib to
reconstruct each segment before yielding data back to ssync. Essentially what
this means is that data is buffered, in memory, on a per segment basis at the
node performing reconstruction and each segment is dynamically reconstructed and
delivered to `ssync_sender` where the `send_put()` method will ship them on
over. The sender is then responsible for deleting the objects as they are sent
in the case of data reversion.
The Auditor
-----------
Because the auditor already operates on a per storage policy basis, there are no
specific auditor changes associated with EC. Each EC archive looks like, and is
treated like, a regular object from the perspective of the auditor. Therefore,
if the auditor finds bit-rot in an EC archive, it simply quarantines it and the
reconstructor will take care of the rest just as the replicator does for
replication policies.

274
doc/source/overview_policies.rst
View File

@ -8,22 +8,22 @@ feature is implemented throughout the entire code base so it is an important
concept in understanding Swift architecture.
As described in :doc:`overview_ring`, Swift uses modified hashing rings to
determine where data should reside in the cluster. There is a separate ring
for account databases, container databases, and there is also one object
ring per storage policy. Each object ring behaves exactly the same way
and is maintained in the same manner, but with policies, different devices
can belong to different rings with varying levels of replication. By supporting
multiple object rings, Swift allows the application and/or deployer to
essentially segregate the object storage within a single cluster. There are
many reasons why this might be desirable:
determine where data should reside in the cluster. There is a separate ring for
account databases, container databases, and there is also one object ring per
storage policy. Each object ring behaves exactly the same way and is maintained
in the same manner, but with policies, different devices can belong to different
rings. By supporting multiple object rings, Swift allows the application and/or
deployer to essentially segregate the object storage within a single cluster.
There are many reasons why this might be desirable:
* Different levels of replication: If a provider wants to offer, for example,
2x replication and 3x replication but doesn't want to maintain 2 separate clusters,
they would setup a 2x policy and a 3x policy and assign the nodes to their
respective rings.
* Different levels of durability: If a provider wants to offer, for example,
2x replication and 3x replication but doesn't want to maintain 2 separate
clusters, they would setup a 2x and a 3x replication policy and assign the
nodes to their respective rings. Furthermore, if a provider wanted to offer a
cold storage tier, they could create an erasure coded policy.
* Performance: Just as SSDs can be used as the exclusive members of an account or
database ring, an SSD-only object ring can be created as well and used to
* Performance: Just as SSDs can be used as the exclusive members of an account
or database ring, an SSD-only object ring can be created as well and used to
implement a low-latency/high performance policy.
* Collecting nodes into group: Different object rings may have different
@ -36,10 +36,12 @@ many reasons why this might be desirable:
.. note::
Today, choosing a different storage policy allows the use of different
object rings, but future policies (such as Erasure Coding) will also
change some of the actual code paths when processing a request. Also note
that Diskfile refers to backend object storage plug-in architecture.
Today, Swift supports two different policy types: Replication and Erasure
Code. Erasure Code policy is currently a beta release and should not be
used in a Production cluster. See :doc:`overview_erasure_code` for details.
Also note that Diskfile refers to backend object storage plug-in
architecture. See :doc:`development_ondisk_backends` for details.
-----------------------
Containers and Policies
@ -61,31 +63,33 @@ Policy-0 is considered the default). We will be covering the difference
between default and Policy-0 in the next section.
Policies are assigned when a container is created. Once a container has been
assigned a policy, it cannot be changed (unless it is deleted/recreated). The implications
on data placement/movement for large datasets would make this a task best left for
applications to perform. Therefore, if a container has an existing policy of,
for example 3x replication, and one wanted to migrate that data to a policy that specifies
a different replication level, the application would create another container
specifying the other policy name and then simply move the data from one container
to the other. Policies apply on a per container basis allowing for minimal application
awareness; once a container has been created with a specific policy, all objects stored
in it will be done so in accordance with that policy. If a container with a
specific name is deleted (requires the container be empty) a new container may
be created with the same name without any restriction on storage policy
enforced by the deleted container which previously shared the same name.
assigned a policy, it cannot be changed (unless it is deleted/recreated). The
implications on data placement/movement for large datasets would make this a
task best left for applications to perform. Therefore, if a container has an
existing policy of, for example 3x replication, and one wanted to migrate that
data to an Erasure Code policy, the application would create another container
specifying the other policy parameters and then simply move the data from one
container to the other. Policies apply on a per container basis allowing for
minimal application awareness; once a container has been created with a specific
policy, all objects stored in it will be done so in accordance with that policy.
If a container with a specific name is deleted (requires the container be empty)
a new container may be created with the same name without any restriction on
storage policy enforced by the deleted container which previously shared the
same name.
Containers have a many-to-one relationship with policies meaning that any number
of containers can share one policy. There is no limit to how many containers can use
a specific policy.
of containers can share one policy. There is no limit to how many containers
can use a specific policy.
The notion of associating a ring with a container introduces an interesting scenario:
What would happen if 2 containers of the same name were created with different
Storage Policies on either side of a network outage at the same time? Furthermore,
what would happen if objects were placed in those containers, a whole bunch of them,
and then later the network outage was restored? Well, without special care it would
be a big problem as an application could end up using the wrong ring to try and find
an object. Luckily there is a solution for this problem, a daemon known as the
Container Reconciler works tirelessly to identify and rectify this potential scenario.
The notion of associating a ring with a container introduces an interesting
scenario: What would happen if 2 containers of the same name were created with
different Storage Policies on either side of a network outage at the same time?
Furthermore, what would happen if objects were placed in those containers, a
whole bunch of them, and then later the network outage was restored? Well,
without special care it would be a big problem as an application could end up
using the wrong ring to try and find an object. Luckily there is a solution for
this problem, a daemon known as the Container Reconciler works tirelessly to
identify and rectify this potential scenario.
--------------------
Container Reconciler
@ -184,9 +188,9 @@ this case we would not use the default as it might not have the same
policy as legacy containers. When no other policies are defined, Swift
will always choose ``Policy-0`` as the default.
In other words, default means "create using this policy if nothing else is specified"
and ``Policy-0`` means "use the legacy policy if a container doesn't have one" which
really means use ``object.ring.gz`` for lookups.
In other words, default means "create using this policy if nothing else is
specified" and ``Policy-0`` means "use the legacy policy if a container doesn't
have one" which really means use ``object.ring.gz`` for lookups.
.. note::
@ -244,17 +248,19 @@ not mark the policy as deprecated to all nodes.
Configuring Policies
--------------------
Policies are configured in ``swift.conf`` and it is important that the deployer have a solid
understanding of the semantics for configuring policies. Recall that a policy must have
a corresponding ring file, so configuring a policy is a two-step process. First, edit
your ``/etc/swift/swift.conf`` file to add your new policy and, second, create the
corresponding policy object ring file.
Policies are configured in ``swift.conf`` and it is important that the deployer
have a solid understanding of the semantics for configuring policies. Recall
that a policy must have a corresponding ring file, so configuring a policy is a
two-step process. First, edit your ``/etc/swift/swift.conf`` file to add your
new policy and, second, create the corresponding policy object ring file.
See :doc:`policies_saio` for a step by step guide on adding a policy to the SAIO setup.
See :doc:`policies_saio` for a step by step guide on adding a policy to the SAIO
setup.
Note that each policy has a section starting with ``[storage-policy:N]`` where N is the
policy index. There's no reason other than readability that these be sequential but there
are a number of rules enforced by Swift when parsing this file:
Note that each policy has a section starting with ``[storage-policy:N]`` where N
is the policy index. There's no reason other than readability that these be
sequential but there are a number of rules enforced by Swift when parsing this
file:
* If a policy with index 0 is not declared and no other policies defined,
Swift will create one
@ -269,9 +275,11 @@ are a number of rules enforced by Swift when parsing this file:
* The policy name 'Policy-0' can only be used for the policy with index 0
* If any policies are defined, exactly one policy must be declared default
* Deprecated policies cannot be declared the default
* If no ``policy_type`` is provided, ``replication`` is the default value.
The following is an example of a properly configured ``swift.conf`` file. See :doc:`policies_saio`
for full instructions on setting up an all-in-one with this example configuration.::
The following is an example of a properly configured ``swift.conf`` file. See
:doc:`policies_saio` for full instructions on setting up an all-in-one with this
example configuration.::
[swift-hash]
# random unique strings that can never change (DO NOT LOSE)
@ -280,10 +288,12 @@ for full instructions on setting up an all-in-one with this example configuratio
[storage-policy:0]
name = gold
policy_type = replication
default = yes
[storage-policy:1]
name = silver
policy_type = replication
deprecated = yes
Review :ref:`default-policy` and :ref:`deprecate-policy` for more
@ -300,11 +310,14 @@ There are some other considerations when managing policies:
the desired policy section, but a deprecated policy may not also
be declared the default, and you must specify a default - so you
must have policy which is not deprecated at all times.
* The option ``policy_type`` is used to distinguish between different
policy types. The default value is ``replication``. When defining an EC
policy use the value ``erasure_coding``.
* The EC policy has additional required parameters. See
:doc:`overview_erasure_code` for details.
There will be additional parameters for policies as new features are added
(e.g., Erasure Code), but for now only a section name/index and name are
required. Once ``swift.conf`` is configured for a new policy, a new ring must be
created. The ring tools are not policy name aware so it's critical that the
Once ``swift.conf`` is configured for a new policy, a new ring must be created.
The ring tools are not policy name aware so it's critical that the
correct policy index be used when creating the new policy's ring file.
Additional object rings are created in the same manner as the legacy ring
except that '-N' is appended after the word ``object`` where N matches the
@ -404,43 +417,47 @@ Middleware
----------
Middleware can take advantage of policies through the :data:`.POLICIES` global
and by importing :func:`.get_container_info` to gain access to the policy
index associated with the container in question. From the index it
can then use the :data:`.POLICIES` singleton to grab the right ring. For example,
and by importing :func:`.get_container_info` to gain access to the policy index
associated with the container in question. From the index it can then use the
:data:`.POLICIES` singleton to grab the right ring. For example,
:ref:`list_endpoints` is policy aware using the means just described. Another
example is :ref:`recon` which will report the md5 sums for all of the rings.
Proxy Server
------------
The :ref:`proxy-server` module's role in Storage Policies is essentially to make sure the
correct ring is used as its member element. Before policies, the one object ring
would be instantiated when the :class:`.Application` class was instantiated and could
be overridden by test code via init parameter. With policies, however, there is
no init parameter and the :class:`.Application` class instead depends on the :data:`.POLICIES`
global singleton to retrieve the ring which is instantiated the first time it's
needed. So, instead of an object ring member of the :class:`.Application` class, there is
an accessor function, :meth:`~.Application.get_object_ring`, that gets the ring from :data:`.POLICIES`.
The :ref:`proxy-server` module's role in Storage Policies is essentially to make
sure the correct ring is used as its member element. Before policies, the one
object ring would be instantiated when the :class:`.Application` class was
instantiated and could be overridden by test code via init parameter. With
policies, however, there is no init parameter and the :class:`.Application`
class instead depends on the :data:`.POLICIES` global singleton to retrieve the
ring which is instantiated the first time it's needed. So, instead of an object
ring member of the :class:`.Application` class, there is an accessor function,
:meth:`~.Application.get_object_ring`, that gets the ring from
:data:`.POLICIES`.
In general, when any module running on the proxy requires an object ring, it
does so via first getting the policy index from the cached container info. The
exception is during container creation where it uses the policy name from the
request header to look up policy index from the :data:`.POLICIES` global. Once the
proxy has determined the policy index, it can use the :meth:`~.Application.get_object_ring` method
described earlier to gain access to the correct ring. It then has the responsibility
of passing the index information, not the policy name, on to the back-end servers
via the header ``X-Backend-Storage-Policy-Index``. Going the other way, the proxy also
strips the index out of headers that go back to clients, and makes sure they only
see the friendly policy names.
request header to look up policy index from the :data:`.POLICIES` global. Once
the proxy has determined the policy index, it can use the
:meth:`~.Application.get_object_ring` method described earlier to gain access to
the correct ring. It then has the responsibility of passing the index
information, not the policy name, on to the back-end servers via the header ``X
-Backend-Storage-Policy-Index``. Going the other way, the proxy also strips the
index out of headers that go back to clients, and makes sure they only see the
friendly policy names.
On Disk Storage
---------------
Policies each have their own directories on the back-end servers and are identified by
their storage policy indexes. Organizing the back-end directory structures by policy
index helps keep track of things and also allows for sharing of disks between policies
which may or may not make sense depending on the needs of the provider. More
on this later, but for now be aware of the following directory naming convention:
Policies each have their own directories on the back-end servers and are
identified by their storage policy indexes. Organizing the back-end directory
structures by policy index helps keep track of things and also allows for
sharing of disks between policies which may or may not make sense depending on
the needs of the provider. More on this later, but for now be aware of the
following directory naming convention:
* ``/objects`` maps to objects associated with Policy-0
* ``/objects-N`` maps to storage policy index #N
@ -466,19 +483,19 @@ policy index and leaves the actual directory naming/structure mechanisms to
:class:`.Diskfile` being used will assure that data is properly located in the
tree based on its policy.
For the same reason, the :ref:`object-updater` also is policy aware. As previously
described, different policies use different async pending directories so the
updater needs to know how to scan them appropriately.
For the same reason, the :ref:`object-updater` also is policy aware. As
previously described, different policies use different async pending directories
so the updater needs to know how to scan them appropriately.
The :ref:`object-replicator` is policy aware in that, depending on the policy, it may have to
do drastically different things, or maybe not. For example, the difference in
handling a replication job for 2x versus 3x is trivial; however, the difference in
handling replication between 3x and erasure code is most definitely not. In
fact, the term 'replication' really isn't appropriate for some policies
like erasure code; however, the majority of the framework for collecting and
processing jobs is common. Thus, those functions in the replicator are
leveraged for all policies and then there is policy specific code required for
each policy, added when the policy is defined if needed.
The :ref:`object-replicator` is policy aware in that, depending on the policy,
it may have to do drastically different things, or maybe not. For example, the
difference in handling a replication job for 2x versus 3x is trivial; however,
the difference in handling replication between 3x and erasure code is most
definitely not. In fact, the term 'replication' really isn't appropriate for
some policies like erasure code; however, the majority of the framework for
collecting and processing jobs is common. Thus, those functions in the
replicator are leveraged for all policies and then there is policy specific code
required for each policy, added when the policy is defined if needed.
The ssync functionality is policy aware for the same reason. Some of the
other modules may not obviously be affected, but the back-end directory
@ -487,25 +504,26 @@ parameter. Therefore ssync being policy aware really means passing the
policy index along. See :class:`~swift.obj.ssync_sender` and
:class:`~swift.obj.ssync_receiver` for more information on ssync.
For :class:`.Diskfile` itself, being policy aware is all about managing the back-end
structure using the provided policy index. In other words, callers who get
a :class:`.Diskfile` instance provide a policy index and :class:`.Diskfile`'s job is to keep data
separated via this index (however it chooses) such that policies can share
the same media/nodes if desired. The included implementation of :class:`.Diskfile`
lays out the directory structure described earlier but that's owned within
:class:`.Diskfile`; external modules have no visibility into that detail. A common
function is provided to map various directory names and/or strings
based on their policy index. For example :class:`.Diskfile` defines :func:`.get_data_dir`
which builds off of a generic :func:`.get_policy_string` to consistently build
policy aware strings for various usage.
For :class:`.Diskfile` itself, being policy aware is all about managing the
back-end structure using the provided policy index. In other words, callers who
get a :class:`.Diskfile` instance provide a policy index and
:class:`.Diskfile`'s job is to keep data separated via this index (however it
chooses) such that policies can share the same media/nodes if desired. The
included implementation of :class:`.Diskfile` lays out the directory structure
described earlier but that's owned within :class:`.Diskfile`; external modules
have no visibility into that detail. A common function is provided to map
various directory names and/or strings based on their policy index. For example
:class:`.Diskfile` defines :func:`.get_data_dir` which builds off of a generic
:func:`.get_policy_string` to consistently build policy aware strings for
various usage.
Container Server
----------------
The :ref:`container-server` plays a very important role in Storage Policies, it is
responsible for handling the assignment of a policy to a container and the
prevention of bad things like changing policies or picking the wrong policy
to use when nothing is specified (recall earlier discussion on Policy-0 versus
The :ref:`container-server` plays a very important role in Storage Policies, it
is responsible for handling the assignment of a policy to a container and the
prevention of bad things like changing policies or picking the wrong policy to
use when nothing is specified (recall earlier discussion on Policy-0 versus
default).
The :ref:`container-updater` is policy aware, however its job is very simple, to
@ -538,19 +556,19 @@ migrated to be fully compatible with the post-storage-policy queries without
having to fall back and retry queries with the legacy schema to service
container read requests.
The :ref:`container-sync-daemon` functionality only needs to be policy aware in that it
accesses the object rings. Therefore, it needs to pull the policy index
out of the container information and use it to select the appropriate
object ring from the :data:`.POLICIES` global.
The :ref:`container-sync-daemon` functionality only needs to be policy aware in
that it accesses the object rings. Therefore, it needs to pull the policy index
out of the container information and use it to select the appropriate object
ring from the :data:`.POLICIES` global.
Account Server
--------------
The :ref:`account-server`'s role in Storage Policies is really limited to reporting.
When a HEAD request is made on an account (see example provided earlier),
the account server is provided with the storage policy index and builds
the ``object_count`` and ``byte_count`` information for the client on a per
policy basis.
The :ref:`account-server`'s role in Storage Policies is really limited to
reporting. When a HEAD request is made on an account (see example provided
earlier), the account server is provided with the storage policy index and
builds the ``object_count`` and ``byte_count`` information for the client on a
per policy basis.
The account servers are able to report per-storage-policy object and byte
counts because of some policy specific DB schema changes. A policy specific
@ -564,23 +582,23 @@ pre-storage-policy accounts by altering the DB schema and populating the
point in time.
The per-storage-policy object and byte counts are not updated with each object
PUT and DELETE request, instead container updates to the account server are performed
asynchronously by the ``swift-container-updater``.
PUT and DELETE request, instead container updates to the account server are
performed asynchronously by the ``swift-container-updater``.
.. _upgrade-policy:
Upgrading and Confirming Functionality
--------------------------------------
Upgrading to a version of Swift that has Storage Policy support is not difficult,
in fact, the cluster administrator isn't required to make any special configuration
changes to get going. Swift will automatically begin using the existing object
ring as both the default ring and the Policy-0 ring. Adding the declaration of
policy 0 is totally optional and in its absence, the name given to the implicit
policy 0 will be 'Policy-0'. Let's say for testing purposes that you wanted to take
an existing cluster that already has lots of data on it and upgrade to Swift with
Storage Policies. From there you want to go ahead and create a policy and test a
few things out. All you need to do is:
Upgrading to a version of Swift that has Storage Policy support is not
difficult, in fact, the cluster administrator isn't required to make any special
configuration changes to get going. Swift will automatically begin using the
existing object ring as both the default ring and the Policy-0 ring. Adding the
declaration of policy 0 is totally optional and in its absence, the name given
to the implicit policy 0 will be 'Policy-0'. Let's say for testing purposes
that you wanted to take an existing cluster that already has lots of data on it
and upgrade to Swift with Storage Policies. From there you want to go ahead and
create a policy and test a few things out. All you need to do is:
#. Upgrade all of your Swift nodes to a policy-aware version of Swift
#. Define your policies in ``/etc/swift/swift.conf``

52
doc/source/overview_replication.rst
View File

@ -111,11 +111,53 @@ Another improvement planned all along the way is separating the local disk
structure from the protocol path structure. This separation will allow ring
resizing at some point, or at least ring-doubling.
FOR NOW, IT IS NOT RECOMMENDED TO USE SSYNC ON PRODUCTION CLUSTERS. Some of us
will be in a limited fashion to look for any subtle issues, tuning, etc. but
generally ssync is an experimental feature. In its current implementation it is
probably going to be a bit slower than RSync, but if all goes according to plan
it will end up much faster.
Note that for objects being stored with an Erasure Code policy, the replicator
daemon is not involved. Instead, the reconstructor is used by Erasure Code
policies and is analogous to the replicator for Replication type policies.
See :doc:`overview_erasure_code` for complete information on both Erasure Code
support as well as the reconstructor.
----------
Hashes.pkl
----------
The hashes.pkl file is a key element for both replication and reconstruction
(for Erasure Coding). Both daemons use this file to determine if any kind of
action is required between nodes that are participating in the durability
scheme. The file itself is a pickled dictionary with slightly different
formats depending on whether the policy is Replication or Erasure Code. In
either case, however, the same basic information is provided between the
nodes. The dictionary contains a dictionary where the key is a suffix
directory name and the value is the MD5 hash of the directory listing for
that suffix. In this manner, the daemon can quickly identify differences
between local and remote suffix directories on a per partition basis as the
scope of any one hashes.pkl file is a partition directory.
For Erasure Code policies, there is a little more information required. An
object's hash directory may contain multiple fragments of a single object in
the event that the node is acting as a handoff or perhaps if a rebalance is
underway. Each fragment of an object is stored with a fragment index, so
the hashes.pkl for an Erasure Code partition will still be a dictionary
keyed on the suffix directory name, however, the value is another dictionary
keyed on the fragment index with subsequent MD5 hashes for each one as
values. Some files within an object hash directory don't require a fragment
index so None is used to represent those. Below are examples of what these
dictionaries might look like.
Replication hashes.pkl::
{'a43': '72018c5fbfae934e1f56069ad4425627',
'b23': '12348c5fbfae934e1f56069ad4421234'}
Erasure Code hashes.pkl::
{'a43': {None: '72018c5fbfae934e1f56069ad4425627',
2: 'b6dd6db937cb8748f50a5b6e4bc3b808'},
'b23': {None: '12348c5fbfae934e1f56069ad4421234',
1: '45676db937cb8748f50a5b6e4bc34567'}}
-----------------------------